Ultrafast and ultrasensitive hydrogen sensors based on self-assembly monolayer promoted 2-dimensional palladium nanoclusters

ABSTRACT

A device and method of making same. The device or hydrogen detector has a non-conducting substrate with a metal film capable of absorbing hydrogen to form a stable metal hydride. The metal film is on the threshold of percolation and is connected to mechanism for sensing a change in electrical resistance in response to the presence of hydrogen in contact with the metal film which causes an increase in conductivity.

RELATED APPLICATIONS

This is a divisional application under 37 C.F.R. §1.53, of applicationSer. No. 11/001,193 filed on Dec. 1, 2004 now U.S. Pat. No. 7,171,841.

CONTRACTUAL ORIGIN OF THE INVENTION

This work is supported by the U.S. Department of Energy (DOE), EnergyEfficiency and Renewable Energy, as part of a DOE program to developelectric power technology, under Contract W-31-109-Eng-38.

FIELD OF THE INVENTION

The invention relates to devices sensitive to hydrogen and moreparticularly to hydrogen gas sensors fabricated from a film of mobilemetal nanoclusters on the threshold of percolation.

BACKGROUND OF THE INVENTION

Hydrogen is an extremely clean energy source for use in fuel cells andinternal combustion engines. However, widespread use of hydrogen as afuel will require innovations in hydrogen storage and hydrogen sensing.Reliable, cheap, compact, and safe hydrogen sensors are needed both formeasuring the hydrogen concentration in flowing gas streams and formonitoring ambient air for leaked hydrogen. It is essential that “alarm”sensors detect hydrogen at a concentration well below the lowerexplosion limit in air of 4%.

The vast majority of hydrogen sensors use a palladium element toselectively absorb hydrogen. Such sensors operate by detecting a changein the properties of the palladium/hydrogen solution relative to thoseof pure palladium. The properties detected include mass, volume,electrical resistivity, optical constants, and the work function.Conventional palladium-based hydrogen sensors, however, have two maindisadvantages: First, the response time for these devices, which tendsto range from several minutes to 0.5 s, is too slow to permit ANL 04-077(E&D 313) useful, real-time monitoring of flowing gas streams. Second,palladium is poisoned by exposure to reactive species, such ashydrocarbons, O₂, H₂O, and CO, that chemisorb on the palladium surfaceand block adsorption sites needed for hydrogen. These species areexactly the sorts of contaminants that are likely to be present in thegaseous feed stream supplying a fuel cell or an internal combustionengine. Exposure of a palladium-based hydrogen sensor to these gasescauses the response time for the sensor to increase, and can necessitaterecalibration of the sensor for hydrogen.

Today, most hydrogen gas sensors are macroscopic palladiumresistor-based sensors. Exposure to hydrogen gas causes an increase inthe resistance in these devices by a factor of up to 1.8 at 25° C. Theresistance increase is caused by the increased resistivity of palladiumhydride relative to pure palladium. Although useful, these sensors notonly suffer from the disadvantages noted above, they tend to requireheating to operate efficiently, which tends to result in higher powerconsumption.

In view of such devices, it is desirable to provide a device sensitiveto hydrogen, particularly a hydrogen gas sensor that is very easy tofabricate and responds very quickly to the presence of hydrogen gas.

SUMMARY OF THE INVENTION

This invention relates to hydrogen sensitive devices and moreparticularly to a hydrogen sensor. More specifically this inventionrelates to a hydrogen sensor with a rapid response time (<1 second),ultrahigh sensitivity and to a method of making the sensor. Shortresponse time hydrogen sensors is needed for measuring the hydrogenconcentration in flowing gas streams and for monitoring ambient air forleaked hydrogen.

The ultrafast response and ultrasensitive hydrogen sensor of thisinvention consists of a discontinuous palladium film comprised ofnanoclusters which are not or are barely connected to each other in theabsence of hydrogen, on various substrates. These films are on thethreshold of percolation. The hydrogen sensing of the palladiumnanoclusters is based on the increase in conductance induced by the sizeincrease of the palladium nanoclusters in the presence of hydrogen. Theswelling leads to a higher electrical conductivity by increasing thenumber of contacts between neighboring nanoclusters. Nanoclusterhydrogen sensor systems have been developed by thermally depositing afew nanometers of palladium onto flat oxide substrates such as glass orsilicon dioxide that are preferably coated with a siloxane self-assemblymonolayer. As palladium is deposited onto such hydrophobic flatsurfaces, palladium nanoclusters with only a few nanometers intervalsapart from each other spontaneously form or bead in order to reach theminimal surface free-energy. In addition, the siloxane self-assembledmonolayer in between the palladium nanoclusters and the oxide substratealso reduces the palladium nanoclusters adhesion on the substrates,which significantly accelerates the swelling of the palladiumnanoclusters. The response time to conductance change of palladiumclusters formed by coating 6 nm palladium on a siloxane treated glasssubstrate, subjected to 2% hydrogen mixed with 98% nitrogen was 68milliseconds with a remarkable conductance signal change of 100%. Theinventive device can detect hydrogen concentration as low as 20 ppm (1ppm is one part per million), i.e. 0.002% hydrogen, which is 2,000 timesbelow the explosion limitation of hydrogen (4%).

Accordingly, an object of the present invention is to provide a device,comprising a non-conducting substrate having thereon a metal filmcapable of absorbing hydrogen to form a stable metal hydride, the metalfilm being on the threshold of percolation, and mechanism in electricalcommunication with the metal film for sensing a change in electricalresistance in response to the presence of hydrogen in contact with themetal film, whereby hydrogen absorbed by the metal film forms a metalhydride larger in volume than the metal resulting in percolation of themetal film and an increase in the conductivity thereof.

Another object of the present invention is to provide a hydrogendetector, comprising a non-conducting substrate having associatedtherewith mobile Pd or Pd alloy nanoclusters forming a film on thethreshold of percolation, and mechanism in electrical communication withthe Pd or Pd alloy film for sensing the change in electrical resistancein response to the presence of hydrogen in contact with the Pd or Pdalloy film, whereby hydrogen absorbed by the Pd or Pd alloy nanoclusterscauses the nanoclusters to swell resulting in percolation of the Pd orPd alloy film and an increase in the conductivity thereof.

A final object of the present invention is to provide a method of makinga device, comprising providing a non-conducting substrate, depositing ametal film capable of absorbing hydrogen to form a stable metal hydrideon or in association with the substrate, the metal film being on thethreshold of percolation, and establishing mechanism in electricalcommunication with the metal film for sensing a change in electricalresistance in response to the presence of hydrogen in contact with themetal film, whereby hydrogen absorbed by the metal film forms a metalhydride larger in volume than the metal resulting in percolation of themetal film and an increase in the conductivity thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention consists of certain novel features and a combination ofparts hereinafter fully described, illustrated in the accompanyingdrawings, and particularly pointed out in the appended claims, it beingunderstood that various changes in the details may be made withoutdeparting from the spirit, or sacrificing any of the advantages of thepresent invention.

FIG. 1A is a graphical representation of the relationship between thethickness and conductivity of a film deposited on a clean glass surfaceand a siloxane self assembled monolayer coating glass surface;

FIG. 1B is an Atomic Forced Microscopy (AFM) of a nominal 3.3 nm Pdevaporated on a clean glass substrate;

FIG. 1C is an AFM of nominal 3.3 nm Pd evaporated on a siloxane coatedglass substrate;

FIG. 2A is a graphical representation of the relationship betweenconductance and elapsed time for a 3.3 nanometer Pd film on SAM siloxanein a 2% hydrogen and nitrogen mixture;

FIG. 2B is a plot of 50 exposures to hydrogen of the sample used in FIG.2A;

FIG. 3A is an AFM image of a 3.3 nm on a SAM siloxane coated glass;

FIG. 3B is an AFM image of the same glass illustrated in FIG. 3A in ahydrogen gas atmosphere;

FIG. 4 is a graphical representation of the relationship betweenconductance and time for a 3.3 nm Pd film deposited on a clean glasssubstrate with a 2% hydrogen and nitrogen mixture;

FIG. 5A is a sensitivity plot showing the relationship of a single 3.3nm Pd on siloxane SAM for various concentrations of hydrogen with thebalance being nitrogen;

FIG. 5B is a plot like FIG. 5A showing the comparison of conductancechange and percentage versus the hydrogen concentration in percentagefor a 3.3 nm thick paladium film on a SAM monolayer and a 10 nm Pd filmon a SAM monolayer;

FIG. 6 is a representation of electron conductive paths in a unmodifiedglass having Pd grains deposited thereon; and

FIG. 7 is a representation of the glass of FIG. 6 with a siloxane layershowing the affect thereof with Pd beads formed thereon.

DESCRIPTION OF THE PREFERRED EMBODIMENT

We investigated the transport response to hydrogen in ultrathin Pd filmswith nanoscopic surface structure and high surface-volume ratio, becausethe electronic property in ultrathin metal film can be highly surfacemorphology-dependent, particularly at the percolation threshold. Thescattering of conduction electrons at grain boundaries or at planarinterfaces defined by the top and bottom surfaces of the ultrathin filmcan influence significantly to the film conductivity. This inventionrelates to transport transition upon hydrogen adsorption in ultrathin Pdfilm preferably deposited on a siloxane self-assembled monolayer (SAM)coated glass. The resulting ultrathin Pd film, preferably less thanabout 10 nm, appears refined and densely packed Pd nanograins andexhibits reversible fast and sensitive response to the presence ofhydrogen.

The device fabrication involves two steps. First, sanitized microscopecover glass pieces were immersed in 1 mM N-octyldimethylchlorosilane ina 4:1 volume mixture of hexadecane and chloroform or in commercial rainrepellant for 24 hrs for the formation of siloxane SAM and rinsed withisopropanol. Ultrathin Pd was then evaporated onto the siloxane SAMcoated glass and the nominal thickness was recorded by a quartz crystalmicrobalance. For characterization and calibration, two gold contacts(0.3 mm apart) were pre-deposited onto the two sides of the substrateand connected to the outside circuit through a vacuum feedthrough inorder for in-situ measuring the conductivity of the ultrathin Pd filmduring its growth.

In another example, the following procedure was used to make a hydrogensensor.

1) Microscope cover glass slips (1×1 inch, 120 micron in thickness, GoldSeal Record No. 3306) were first sanitized by 7X detergent, rinsed withD.I water and then washed with pure acetone under ultrasonication andthen dried under a stream of nitrogen gas. The cover glass was then cutinto 2 mm×5 mm pieces and were immersed in commercial rain repellant(Prestone, * * *) or in 1 mM N-octyldimethylchlorosilane (Unitedchemical Technology, Lot #20400028) in a 4:1 volume mixture ofhexadecane and chloroform for 24 hrs for the formation of siloxane SAMand rinsed with isopropanol. These were dried in a stream of nitrogen.

2) Two 50 nm thick gold contacts were deposited onto the substrates witha 20-300 micron wide gap across the exposed face of the glass piece.

3) Deposition of about 3.5 nm Pd (Aldrich, 99.99%, * * * ) were coatedonto the substrate by an Polaron E6700, Turbo Vacuum Evaporator (VGMicrotech), The power switch was slowly turned to level “4”. The vacuumis less than 2×10⁻⁵ mbar. The distance between substrate and evaporationsource is approximately 10 cm while the thickness monitor is 35 o tiltedfrom vertical direction. The 3.5 nm thickness is observed by the quartzcrystal thickness monitor and the quartz crystal is newly installed.Note that the 3.5 nm is only suitable for the setup in our lab andshould be recalibrated if other system will be used. This thickness onour system is the threshold for percolation as described on slide #24.The measurement by thickness monitor is really irrelevant since itdepends on the thickness monitor's position, angle shielding, etc. Theimportant parameter is being at the threshold of percolation.

4) The Pd coated glass gets glued with cyanoacrylate Pd side up onto a 5mm×13 mm fiberglass filled printed circuit board which has simple tincoated copper lines for contacts. The two gold contacts on the Pd coatedglass get connected to on the board using a small piece of indiumpressed onto both the gold and printed circuit board lines. Indium inthis design gives a soft but robust electrical contact. In a commercialdesign the Pd would be deposited directly onto a carrier substrate. Theprinted circuit board gives a standard geometry and is easy to handle sothat testing can be completed without having to make and break delicatecontacts.

Testing Process

A special cell was designed to provide continual flow of both purgingand test gases with a small dead volume so accurate response times couldbe measured. The concentrations are set using two Aalbourg mass flowcontrollers and a manifold comprised of 5 solenoid valves. The finalvalve before the sensor delivers a purge gas of nitrogen when in thenormally closed position or the pre-mixed test gas if in the openposition. The response time of this valve is stated from themanufacturer as a maximum of 25 ms on and 30 ms max off. The dead volumeand flow rates used give an addition 3 ms between the valve activationand the measuring of the signal.

Experiments are programmed using Labview to control the valves and aCypress Systems Omni 90 potentiostat. The interface to the computer is aNational Instruments PCMCIA 6064E data acquisition card and some customelectronics. The potentiostat is programmed to apply a voltage andmeasure a current. The current is converted to a resistance by therelationship V=1R where V is the potential in Volts, I is the currentand R is the resistance in ohms.

FIG. 1A shows the dependence of film conductivity in logarithmic scaleon the nominal film thickness during the Pd evaporation onto a cleanglass substrate and on a siloxane coated glass. For both of the twotypes of substrates, the Pd film conductivity is stronglythickness-dependent within the initial 5 nm deposition and themeasurable film conductivity from 1 nm to 5 nm increased nearly fiveorders of magnitude. In the higher thickness region, the Pd filmconductivity appears less sensitive to film thickness. In detail, acontinuous increase in film conductivity is observed as Pd film grew onthe clean glass and asymptotically approaches to 6.2×10⁵ S·m⁻¹ at 10 nmnominal thickness. In comparison, for Pd film growing on a SAM coatedglass, an obvious kink can clearly be identified at nominal filmthickness between 3.3 and 3.4 mm, which distinguishes it from the cleanglass. Within this nominal thickness growth of 0.1 nm, the conductivityof the film increased by a factor of 45. Above this critical thickness,the two curves starts to converge in higher thickness region. Thesurface morphology of the two types of samples at the critical thicknessof 3.3 nm is also shown with atomic force microscopy (AFM). EvaporatedPd wets the clean glass surface and forms randomly connected Pd domainswith averaged size of 1,200 nm², about 6 nm in height and approximately100 nm apart from each other as seen in FIG. 1B. In contrast, 3.3 nm Pdon siloxane SAM coated glass as seen in FIG. 1C appears as much denserand refined Pd nanoclusters. We believe that the onset kink at ˜3.3 nmin FIG. 1A indicates the film is on the threshold of percolation asshown in FIG. 1C, on which further added Pd atoms begin to fill theboundary and drastically enhance the metallic conduction betweenneighboring grains. The increased surface free energy between siloxaneSAM and Pd significantly reduces the size of the deposited Pd grainsaccompanied by the increased grain density in order to balance the totalmaterials. The direct consequence is that the total amount of theboundaries between the Pd grains increases and the width of the boundarybetween the grains gets narrowed. The average width of grain boundary inthe Pd ultrathin film on siloxane SAM is below 10 nm while that for Pdon clean glass is around 100 nm.

Opposite to the response of a bulk Pd resistor to H₂, the conductance ofa ultrathin Pd film on siloxane SAM increased 65% in the presence of 2%H₂, as seen in FIG. 2A. The corresponding rise-time (baseline to 90%signal saturation) of approximately 70 ms has been observed for theresponse of the film to 2% H₂, a concentration below the hydrogenexplosion range of 4%-75% for effective early-alarming. In addition, theconductance signal intensity and the rising time undergo negligiblechanges after being repeatedly exposed to 2% H₂, as seen in FIG. 2B.

It is known that the conductivity of macroscopic PdHx reaches a minimumat x˜0.7, with a surprising rise following at the further increase of x,presumably due to the further electron filling by hydrogen to the 6-thd-band of Pd more than offsets the loss in the hole number and the neteffect is the increase in total number of charge carriers, i.e electron.However, such rise in conductivity never exceeds the conductivity ofpure Pd while the observed increase in conductance in the presentinvention is relative to pure Pd. Therefore, a different mechanism mustoperate in the SAM-supported ultrathin Pd film at percolation threshold.

In-situ AFM studies is revealed that under a stream of hydrogen, thegrain-boundaries in the Pd film tend to heal and the bearing film volumewithin the scanning area swelled ˜7%, as seen in FIGS. 3A and 3B. It isalso known that hydrogen atoms can diffuse into the FCC Pd cluster thatleads to the dilation of Pd lattice. At room temperature, the latticeconstant of fcc Pd is 3.889 Å, whereas that for stable β-phase PdH_(0.7)is 4.025 Å—an increase of 3.5%, corresponding to 11% increase in volume.Therefore, it is believed that the healing of the grain-boundary in Pdnanocluster film is caused by the dilation of the individual Pdnanograins, which in turn, leads to a higher electrical conductance bythe increasing number of contacts between neighboring Pd nanograins.Such hydrogen-induced conductivity of Pd film increase is maximized whenfilm thickness is at percolation threshold, which can be characterizedby the plot of film conductivity versus film thickness.

The siloxane SAM in between the Pd layer plays crucial dual-roles.Siloxane SAM modifies the surface morphology of the coated Pd film,which considerably reduces the inter-distance between neighboringgrains, increases the amount of the grain boundary and substantiallyenhances the conductivity variation at percolation threshold asindicated by the comparison between FIG. 1A and FIG. 1B. In addition,siloxane SAM also alleviates the stiction between Pd nanograins and theglass substrate, which is so strong that even hydrofluoric acid isfailed to strip Pd film off a glass substrate.

As described above, the adsorption of hydrogen by Pd can have twoopposing effects on the transport properties of Pd thin film, that is,decreasing the film conductance by the formation of PdH, and increasingthe film conductance by healing of the grain boundary in the film. Thecompetition of these two opposing effect can be observed in samples ofthin Pd film deposited on clean glass, as seen in FIG. 4. The initialdrop in conductance ascribed to the formation of Pd hydride is offsetand eventually outpaced by a rising conductance due to Pd clusterswelling. For these samples, the events sequence during conductancevariation always starts with a decrease followed by an increase inconductance, while inversed events sequence has never been observed.

According to the scaling relation for percolation disorder, theconductivity R for percolating systems can be written by the scaling lawC˜|P−P_(c)|⁻, where P is the surface fraction of conducting, Pc is thecritical value of p corresponding to the percolation threshold, and isthe conductivity exponent ( ). Assuming the conducting populationdecrease linearly in time during the desorption of hydrogen, theconductance of film at percolation threshold can be expressed asC˜|t−t_(f)|⁻, where t_(f)=t(P_(c)) is the filling time.

The dual-effect brought by the siloxane SAM yields the H₂ detectingability of the device as sensitive as to 25 ppm, as seen in FIG. 5A. Thesensor exhibited a sigmoidal response curve, as seen in FIG. 5B byincreased conductance to hydrogen. Inversely, a sensor based on 10 nmthick Pd film on siloxane SAM, whose conductivity is near inert to filmthickness, see FIG. 1, shows less sensitivity to the presence ofhydrogen by decreased conductance, as seen in FIG. 5B.

Referring now to FIG. 6, there is illustrated a representation of theeffect of depositing nanometer grains of Pd on an unmodified glasssubstrate. Without this siloxane or other low free surface energysurface, the number of conductive paths will be the same but theresistivity increases when hydrogen is present, thus making a sensor ordevice with less conductivity. The expansion of Pd grains that arealready touching may make some conductive paths rupture and willcontribute to the irreversibility of some sensors due to the rupturingor defamation of some conductive paths. A device made in accordance withFIG. 6, would be useful in determining whether hydrogen was ever presentin the atmosphere to which a sensor made in accordance with FIG. 6 wasexposed, since the reaction here is not as reversible or irreversiblewith respect to devices made as illustrated in FIG. 7.

Referring now to FIG. 7, there is illustrated the mechanism in which amodification of a glass surface is provided wherein a stiction-reducingsurface or coating is on the substrate, for instance a siloxane layerand most preferably, a siloxane monolayer. Alternatively, the substratemay have a hydrophobic surface, as will be described, but in any event,as seen, Pd deposited on such a device forms beads rather than forming abond with the substrate. It is believed that siloxane which provides lowsurface free energy is responsible for the beads forming and this isextremely important because the beads are therefore more mobile in thedevice of FIG. 7 than in the device of FIG. 6 so that when hydrogen isintroduced the beads swell and are able to assume new positions that areoptimized to provide a maximum number of contacts thereby increasing theelectrical conductivity greatly as opposed to the device shown in FIG.6.

Although the device has been principally described with respect to ahydrogen detector, there may be other uses for such a device such asswitches and other devices. It is intended to cover in the claimsappended hereto all such devices.

As previously stated, the Pd nanoclusters may have an average diameterof less than about 10 nanometers and the film thickness may also be andis preferably less than about 10 nanometers and most preferably lessthan about 5 nanometers. In the particular chamber used to deposit thefilms of the subject invention as described hereinbefore, films of 3.3to 3.4 nm showed superior sensitivity but that is a function of thegeometry of the electrodes and substrate and the chamber in which thefilms were made and it is understood by one of ordinary skill in the artthat devices have to be calibrated depending upon a variety of factorsused in the production thereof.

The substrate maybe any suitable material, but preferably has lowsurface free energy. In addition, the substrate itself or the surfacethereof may be hydrophobic or the hydrophobic characteristic may be dueto a coating on the substrate. Available substrates are glass, silicon,various polymers, polypropylene, fluorinated polyethylene, or wax. Thislist is not exhaustive but only representative.

The metal film preferably is in the form of mobile metal nanoclustersbut in the case of untreated glass or other surfaces with higher freesurface energy the nanoclusters may be less mobile and be used toprovide a different type of sensor or device.

Referring to siloxane self assembled layers, preferably a siloxaneself-assembled monolayer is used and by monolayer we mean substantiallymonolayer between it may be at least portions of the self assembledlayer is not a monoloayer. Moreover, preferably, the siloxane is analkylsilane and may contain fluorine atoms for instance such asundecyltrichlorosilane or the alkylsilane may contain chlorine atoms forinstance octadecyl trichlorosilane. Other silanes such as alkylsilaneswhich are useful in the present invention are carboxyterminatedalkylsilanes.

The metal film of nanoclusters maybe one or more of Pd, Cu, Au, Ni, Rh,Pt, Y, La or alloys thereof and most preferably is Pd or alloys thereof.As used in the claims herein, the term “non-conducting substrate” meansa non-electrically conducting substrate although the substrate may be anion conducting substrate.

While there has been disclosed what is considered to be the preferredembodiments of the present invention, it is understood that variouschanges in the details may be made without departing from the spirit, orsacrificing any of the advantages of the present invention.

1. A hydrogen detector, comprising a non-conducting substrate havingmobile Pd or Pd alloy nanoclusters forming a film on said substrate,said film being on the threshold of percolation; a mechanismelectrically connected to said Pd or Pd alloy film for sensing a changein electrical resistance in response to the presence of hydrogen incontact with said Pd or Pd alloy film; whereby as hydrogen is absorbedby said Pd or Pd alloy nanoclusters they swell resulting in an increasein the conductivity thereof and wherein said non-conducting substratehas a stiction-reducing coating thereon.
 2. The device of claim 1,wherein said metal nanoclusters have an average diameter of less thanabout 10 nanometers.
 3. The hydrogen detector of claim 1, wherein saidnon-conducting substrate is glass or a material having a low freesurface energy.
 4. The hydrogen detector of claim 3, wherein saidnon-conducting substrate is polyethylene or polypropylene or wax.
 5. Thehydrogen detector of claim 1, wherein said non-conducting substrate hasa hydrophobic surface.
 6. The hydrogen detector of claim 1, whereinnon-conducting substrate has a self-assembled layer thereon.
 7. Thehydrogen detector of claim 1, wherein said non-conducting substrate hasa siloxane monolayer coating thereon.
 8. The hydrogen detector of claim7, wherein the siloxane is an alkylsilane.
 9. The hydrogen detector ofclaim 1, wherein said metal film on the threshold of percolation has athickness of less than about 10 nm.
 10. The hydrogen detector of claim1, wherein said metal film on the threshold of percolation is less thanabout 5 nanometers thick.
 11. The hydrogen detector of claim 1, whereinsaid metal film on the threshold of percolation is between 3.3 and 3.6nanometers thick.